The invention relates to a system for the preparation of air and to a method for the preparation of air in an aircraft.
Cooled pressurized air is required for the operation of oxygen and nitrogen generation units in aircrafts and for the operation of units in aircrafts with similar demands on the supply air.
The aircraft tanks fill up with a mix of kerosene vapor and air due to the removal of kerosene during the flight. With an unfavorable composition, an explosive mix can be created which ignites due to spontaneous ignition or is ignited by spark formation. To preclude this risk, it is proposed to reduce the oxygen content of the air in the kerosene tank. Investigations have shown that a spontaneous ignition of the mix can be reliably avoided by a reduction of the (normal) oxygen content of the air in the kerosene tank from 20.9% to approximately 12% to 14.5%. In addition to a low oxygen content, a high nitrogen content is advantageous to prevent a spontaneous ignition of the mix.
A unit is known from EP 1 375 349 A1 for the generation of nitrogen which will be termed an OBIGGS (on board inert gas generation system) in the following. This system uses a molecular sieve, whereby the largest portion of oxygen is filtered. A product gas with a greatly reduced oxygen content and a highly increased nitrogen content is created at the outlet side. This product gas can now be introduced into the kerosene tanks to preclude the risk of an ignition of the kerosene/air mix. Furthermore, the OBIGGS product gas can e.g. also be guided into freight spaces to minimize the risk of fire there.
The OBIGGS requires a supply with cooled pressurized air of approximately 50° C. to 90° C. Depending on the filter technology used and on the system size, the minimum supply pressure required amounts to approximately 1.8 bar (rel.) or 3.1 bar (rel.). According to the prior art, the supply of the OBIGGS takes place in that pre-cooled pressurized air (approximately 200° C. and 2 bar relative pressure) is guided out of the pressurized air bleeding system of the aircraft via a closable inlet valve (OSOV) into a special OBIGGS heat exchanger (OHX) and is there cooled according to the demands. The heat exchanger OHX is located in a ram air passage.
An already known system of this kind can be seen from FIG. 1. The oxygen is then filtered by the corresponding filter technology in the OBIGGS, whereby product gas is obtained with a nitrogen portion of >90%. It is then fed, as described above, to the tank containers and/or to the freight spaces. As can be seen from FIG. 1, an ozone converter is provided which is disposed upstream of the heat exchanger OHX on the pressurized air side. The ozone content of the ambient air and thus of the pressurized air increases with the flight altitude. The ozone can impair the action of the oxygen separation in the OBIGGS. This disadvantageous effect can be diminished or precluded by the ozone converter which reduces the ozone content.
As furthermore visible from FIG. 1, a valve OBPV is provided which is arranged in a bypass line which connects the inlet of the heat exchanger OHX on the pressurized air side with its outlet on the pressurized air side. Hot pressurized air is guided around the heat exchanger OHX by opening the valve OBPV. A specific minimum temperature of the cooled supply air can thereby also be set with cold ambient temperature (ram air temperature). Alternatively, a temperature control is also possible by a flap or a valve (not shown) in the ram air passage, whereby the ram air amount is reduced.
It is mostly necessary for the OBIGGS to be in operation during the whole flight. If the aircraft is on the ground, however, no ram air is present, i.e. the ram air for the cooling of the heat exchanger OHX must be generated otherwise or using corresponding means. This can be achieved, for example, by means of an ejector. Provision is made in this process for bled air to be guided via the open valve OESOV (see FIG. 1) and fed to a nozzle/ejector OEJ. The high discharge speed of the air at the nozzle effects an entrainment of the ambient air, whereby a ram air flow is generated which serves to cool the heat exchanger OHX. The necessary ejector air quantity amounts to approximately 60% of the OBIGGS supply air amount.
Alternatively, it is possible for this purpose for the ram air be generated on the ground by means of a blower in the ram air passage. For this purpose, a power supply and an inspection unit are required which are not shown in FIG. 1.
The following flow rates for the supply of an OBIGGS system are required for an aircraft with approximately 150 passengers:
Cooled OBIGGS pressurized supply air: approx. 100 g/s
Ram air: approx. 150 g/s to 200 g/s
Ejector (ground operation only): approx. 60 g/s.
A total of approximately 160 g/s pressurized air and, additionally, approximately 200 g/s ram air is thus required for the provision of 100 g/s OBIGGS pressurized supply air.
As mentioned above, OBIGGS systems are also known which require a higher supply pressure of at least approximately 3.1 bar (rel.). To balance this, the requirements of cooled pressurized supply air, however, only amount to approximately 35% (in this example thus approximately 35 g/s). Due to the higher pressure and the lower flow rate, this OBIGGS system has a smaller and lighter construction than that which works at lower pressures, but higher flow rates.
One problem in the operation of OBIGGS systems of this kind which require a higher supply pressure of approx. 3.1 bar (rel.) consists of the fact that the level of the pressurized air supply of aircraft lies at approximately 2 bar (rel.) and thus well below the required 3.1 bar (rel.). To provide cooled pressurized air for this type of OBIGGS system, a compressor is thus required. FIG. 2 shows a corresponding architecture which is likewise known from the prior art.
Analog to the system shown in FIG. 1, pressurized air/bled air is supplied to the system via the open valve OSOV. The pressurized air flows through an ozone converter after passing through the valve OSOV. A heat exchanger (OPHX) is disposed upstream of the compressor so that temperatures do not arise in the downstream compressor C which are too high. After passing through the compressor, the pressurized air is cooled to approximately 75° C. in the heat exchanger OMHX and is supplied to the OBIGGS system. The heat exchangers OPHX and OMHX are cooled by ram air, as is shown in FIG. 2. On the ground, the ram air is generated by the ejector OEJ or by blowers. The driving of the compressor takes place by means of a turbine in which pressurized air is expanded which is branched off downstream of the ozone converter. For this purpose, the valve OPRV is opened and pressurized air is supplied to the turbine inlet.
Alternatively to this, the drive of the compressor can also take place by means of an electric motor such as is known from U.S. Pat. No. 4,681,602. The pressurized air for the turbine is thereby saved. However, the electric motor is relatively heavy in construciton and requires further components such as an energy supply (converter) and an inspection unit.
The systems known from the prior art in accordance with FIG. 1 and FIG. 2 are associated with the following disadvantages:                Ram air passage        An independent ram air passage, including the corresponding openings for inlet and outlet in the aircraft structure, is required for the ram air of the heat exchanger OHX (FIG. 1) or OPHX/OMHX (FIG. 2). The available space for freight is thereby reduced and the aircraft weight increased and additional costs are incurred. This disadvantage is made even larger if OBIGGS systems have to be retrofitted in existing aircraft. Fairly large modifications are required in order to subsequently integrate a ram air passage into the aircraft structure with new additional openings for the inlet and outlet.        Ram air flow        For reasons of cost and complexity, a flap at the inlet of the ram air passage is generally dispensed with. The disadvantage arises from this that the maximum ram air passage flow is always provided in flight even if a low ram air flow would be sufficient in specific flight phases or at a low ambient temperature. The ram air flow increases the aircraft kerosene consumption and thus the operating costs.        Ejector/Blower        The OBIGGS operation is generally also required on the ground. Since no ram air is available in this operating state due to lack of ram pressure, it must be actively conveyed. For this purpose, either a blower or, as shown in FIGS. 1 and 2, an ejector (OEJ and OESOV) are installed. Both variations are associated with costs and with weight disadvantages. Furthermore, an additional pressurized air requirement thereby results for the ejector or an additional electrical power requirement, if a ram air blower is alternatively used.        Ozone converter        Already known pressurized air preparation systems mostly require an ozone converter. The pressure losses generated in this process have to be compensated by smaller pressure losses in the following heat exchangers in order to ensure the same supply pressure for the OBIGGS system. The heat exchangers are thereby larger and heavier in construction. A further disadvantage is additionally the weight of the ozone converter.        Availability of pressure during descent        Depending on the required engine power, different pressures are available in the bled air system of the aircraft. The engine power is greatly reduced during the descent/landing approach, with relatively low pressures resulting from this.        The tanks of an aircraft are ventilated such that the same pressure results in the tank as in ambient. The ambient pressure increases during the aircraft descent. To match the internal tank pressure to the ambient pressure, the most air has to be supplied to the tanks in this phase. An OBIGGS system must therefore provide the maximum amount of nitrogen-enriched air in this phase and thus also generates the maximum demands on the OBIGGS pressurized supply air in this phase. Since the performance capability of the systems described in FIG. 1 and in FIG. 2 depends directly on the pressure of the bled air, the performance capability of the OBIGGS pressurized air preparation system is limited just in the phase of maximum demand. This has to be compensated by correspondingly larger dimensioned components, with corresponding disadvantages with respect to weight, construction space and costs resulting from this.        
The disadvantages described above apply equally to the systems from the prior art shown in FIG. 1 and in FIG. 2.
The following additional disadvantages result from the system shown in FIG. 2:
With the system described in FIG. 2, the advantage initially results with respect to the system that higher pressures can be achieved for the OBIGGS supply. The required amount of cooled supply air is thereby substantially reduced and a more efficient OBIGGS system can be used. This advantage is, however, eliminated again for the following reasons:                An additional component is required with the compressor;        a drive is required for the compressor which, in the case of a turbine, requires additional pressurized air from the bled air system. Pressurized air is thus required for the actual OBIGGS supply and for the drive of the turbine and for the ejector for the generation of ram air, i.e. cooling air, in ground operation;        an additional heat exchanger (OPHX) is required, with the corresponding disadvantages with respect to construction space and weight;        due to the smaller required OBIGGS supply air amount (at a higher supply pressure), the required ram air flow could generally be reduced. However, this is negated by the ram air amount which the additional OPHX needs.        